Fix Billboard.computeScreenSpacePosition.
Update CHANGES.md.
Merge branch 'master' into computeScreenSpacePosition

Conflicts:
	CHANGES.md
Share reproject types between layers but not contexts.
Avoid redundant ImageryLayerCollection events.
Fix doc typos.
Update CHANGES.md.
Merge pull request #563 from AnalyticalGraphicsInc/terrainImageryCleanup

Terrain/imagery cleanup
Merge branch 'master' into computeScreenSpacePosition

Conflicts:
	CHANGES.md
	Source/Scene/Billboard.js
	Source/Scene/Label.js
Fix after merge.
Update based on review.
Revert BillboardCollection and update CHANGES.md.
Fix memory leak in Material.
Merge pull request #567 from AnalyticalGraphicsInc/destroyMaterial

Destroy sub-materials.
Add comment to computeScreenSpacePosition
Merge pull request #557 from AnalyticalGraphicsInc/computeScreenSpacePosition

Fix Billboard.computeScreenSpacePosition.
Merge remote-tracking branch 'origin/master' into terrainSample